The day is an extra rare occurrence in which the 13th day of a month lands on a Friday. It's a date that has spurned superstitious attitudes since the Middle Ages. But just how rare is it?

Friday the 13th is when the 13th day of the month in the Gregorian calendar falls on a Friday (well, duh). This happens only at least once every year, so it has become a big deal. Once in a blue ...